Another indicator that the light seals are shot is the spurious "check brake light" feature. If you find the brake light indicator is coming on this is quite often caused by the light cluster being poorly earthed. Any moisture will play havoc with this, indicating that the light seals are allowing moisture through.

Roops.

Apparently, there's a run-off tube that leads to your trunk, on the battery side. I guess water will overflow into your tank if your front sunroof drains are clogged (or something to that effect). Also, check your tail light seals. I was getting water in my trunk too, and that turned out to be the problem. It was about $25 USD for the seals, took 20 minutes to change out.